Abstract: In accordance with the present invention, a controlled biocide feeder limits growth of bacteria (e.g., Legionella pneumophila) in heat transfer water of building air conditioning systems. Preferably, a flow of heat transfer water is directed across a solid biocidal material, such as 1-Bromo-3-chloro-5, 5-dimethylhydantoin (BCDMH) to form a biocidal solution. A biocide control valve responsive to a heat transfer control signal corresponding to an amount of water used in the heat transfer system for passing biocidal solution to the water used in the heat transfer system to provide a biocidal material concentration of less than 12 parts-per-million to prevent corrosion of the air conditioning system. Preferably, the concentration is between about 1.5 and 3 parts-per-million of BCDMH.